Overvoltage protective device



United States Patent 3,391,371 OVERVOLTAGE PROTECTIVE DEVICE Harold E. Wright, Erie, and Edward E. Leofsky, Wesleyville, Pa., assignors to Erie Technological Products, Inc., Erie, Pa., a corporation of Pennsylvania Filed July 6, 1966, Ser. No. 563,182 2 Claims. (Cl. 337-417) This invention is a ceramic overvoltage protective device having an air gap which breaks down to protect equipment from high voltage surges. The construction is simple to make and its reliability is increased by a construction which protects the ceramic from tracking during voltage breakdown. One use is in television receivers for protecting the color tube from voltage surges which may occur at infrequent intervals during the life of the tube.

In the drawing, FIG. 1 is a longitudinal section through the device at an initial stage of manufacture, FIG. 2 is a section on line 2-2 of FIG. 1 and PG. 3 is a longitudinal section through the finished device.

The body of the device comprises a cylinder 1 of suitable ceramic such as steatite coated over its entire exterior surface with a metallic coating 2. The ceramic bodies 1 are conveniently coated in bulk with a nickel or other metallic plating which may be built up to an additional thickness by electroplating copper and then tinned to accept solder. The coating is not necessary on the end surfaces 3 but there is no particular advantage in masking the coating from the end surfaces. At this stage, the ceramic rod merely has an exterior metallized coating 2.

In the second stage of manufacture, a circumferential groove 4 is cut into the ceramic at some point between the ends of the rod. The depth of the groove is many times the thickness of the coating 2 and is at least equal to and usually greater than the axial width of the groove. In a commercial form designed for breakdown in the range of 1000-2000 volts, the depth of the groove was .016" and the width of the groove was .011. After this operation, there is an annular air gap between adjacent ends 5, 6 of the metallized coating 2. This construction confines the voltage breakdown to a spark discharge in air between the ends 5, 6. Erosion of the metal during breakdown merely locally increases the axial length of the gap so that the next protective voltage breakdown occurs at a "ice different point. The discharge is out of contact with the ceramic, so the ceramic is not subject to tracking which could lower the insulation resistance of the ceramic. The air gap between the ends 5, 6 is annular in form, surrounding the entire circumference of the device.

The groove 4 is easily cut by automatic machinery which chucks the ceramic body and moves it against a saw. The axial Width of the groove is easily controlled by the width of the saw. The depth of the groove is also easily controlled and is conveniently held to a value safely in excess of the minimum required.

The manufacture of the device is completed by soldering conventional metal end cap terminas 7 to opposite ends of the body. These terminals have associated leads 8 making electrical connections. The terminal assembly operation is adapted to automation.

We claim:

1. An overvoltage protective device comprising a cylindrical ceramic body having between its ends an inwardly extending circumferential groove of depth at least substantially equal to its axial Width, a pair of metallized coatings overlying the outer surface of the body, said coatings being axially spaced from each other by the groove to provide an annular air gap, said coatings extending respectively from opposite sides of the groove toward opposite ends of the body, the depth of the groove at least substentially equal to the axial width of the groove and being many times the thickness of the coatings whereby the sparking is confined to the air gap between the coatings and does not affect the breakdown voltage by tracking the ceramic.

2. The device of claim 1 in which the body is overcoated with a metallized coating and the circumferential groove is cut through the coating into the body to divide the coating into said pair of axially spaced coatings and to provide a sharply defined annular air gap.

References Cited UNITED STATES PATENTS 2,288,428 6/ 1942 Babler 200-l 15 BERNARD A. GILHEANY, Primary Examiner.

F. E. BELL, Assistant Examiner. 

1. AN OVERVOLTAGE PROTECTIVE DEVICE COMPRISING A CYLINDRICAL CERAMIC BODY HAVING BETWEEN ITS ENDS AN INWARDLY EXTENDING CIRCUMFERENTIAL GROOVE OF DEPTH AT LEAST SUBSTANTIALLY EQUAL TO ITS AXIAL WIDTH, A PAIR OF METALLIZED COATINGS OVERLYING THE OUTER SURFACE OF THE BODY, SAID COATINGS BEING AXIALLY SPACED FROM EACH OTHER BY THE GROOVE TO PROVIDE AN ANNULAR AIR GAP, SAID COATINGS EXTENDING RESPECTIVELY ENDS OF THE BODY, THE DEPTH OF THE GROOVE TOWARD OPPOSITE ENDS OF THE BODY, THE DEPTH OF THE GROOVE AT LEAST SUBSTANTIALLY EQUAL TO THE AXIAL WIDTH OF THE GROOVE AND BEING MANY TIMES THE THICKNESS OF THE COATINGS WHEREBY THE SPARKING IS CONFINED TO THE AIR GAP BETWEEN THE COATINGS AND DOES NOT AFFECT THE BREAKDOWN VOLTAGE BY TRACKING THE CERAMIC. 